1. Field of the Invention
This invention relates generally to a regulated converter circuit with hysteresis and, more particularly, to a regulated d-c to d-c oscillator converter circuit with hysteresis for use in electronic flash devices.
2. Description of the Prior Art
Typical photographic electronic flash devices utilize a battery powered d-c to d-c converter oscillator for charging a flash storage capacitor which may be thereafter selectively discharged through a flashtube to produce a flash of light for illuminating a photographic scene. It is also well known to provide means for automatically controlling the output voltage from the oscillator to the storage capacitor within a desired range in order to achieve a minimum battery drain. One such circuit for this purpose as disclosed in U.S. Pat. No. 3,316,445, entitled "Transistorized Power Supply for a Storage Capacitor with a Regulating Feedback Control" issued Apr. 25, 1967, teaches the use of a neon lamp for feeding back a sample of the output voltage to a switching circuit for controlling the operation of the oscillator. Thus, when the output voltage has reached a desired value, the neon lamp conducts and causes current to flow through it to the switching circuit which, in turn, alters the bias on the oscillator to cause it to terminate operation. When the output voltage of the storage capacitor thereafter discharges to a predetermined value, the neon lamp current is diminished to a critical value and the switching circuit returns to its conductive state so as to apply a suitable bias to the oscillator to cause it to again start oscillating. However, as a result of the inherent instability and high hysteresis of such neon lamps, arrangements employing such lamps have permitted the capacitor voltages to vary between oscillator turn on and turn off by as much as 30% or more. This amounts to an unsatisfactory performance in many instances. Also, the inherently very small hysteresis provided by diodes has made them generally unsatisfactory when employed in the foregoing manner, since this characteristic has resulted in a too frequent on-off cycling of the oscillator.
One such arrangement which overcomes the aforementioned difficulties is disclosed in a U.S. Pat. No. 3,863,128 entitled "Voltage Monitoring Controlling and Protecting Apparatus Employing Programmable Unijunction Transistor", issued Jan. 28, 1975, which teaches various circuit configurations, each of which includes a programmable unijunction transistor in a circuit which compares a voltage to be monitored with a corresponding preset reference voltage and controlls the operation of the power supply in accordance with the results of this comparison. Such a circuit requires two zener diodes in addition to the programmable unijunction transistor as well as additional circuitry which contributes to the overall complexity of the control circuit arrangement.
It is, therefore, an object of this invention to provide a simplified control circuit for effectively terminating the operation of a d-c to d-c converter oscillator when the output voltage thereof increases to a predetermined maximum value and for thereafter restarting the operation of the oscillator when the output voltage decays to a predetermined minimum value.
It is a further object of this invention to provide a simplified control circuit for controlling the operation of a d-c to d-c converter oscillator wherein the control circuit has a desired hysteresis which prevents the oscillator from cycling on and off in too frequent a manner.
It is an even further object of this invention to provide a simplified control circuit for controlling the operation of a d-c to d-c converter oscillator as may be used in a photographic electronic flash device to maintain the output voltage thereof within a desired range in order to protect the storage capacitor from incurring unduly high capacitor voltages while at the same time protecting the battery from being excessively discharged.
It is also an object of this invention to provide a simplified control circuit for controlling the operation of a d-c to d-c converter oscillator as may be used in a photographic electronic flash device in the aforementioned manner without utilizing a programmable unijunction transistor in combination with two zener diodes.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.